I'm going from memory, but I think you have to put the metal spring into the bar and then hook it into the keyboard, then push the bar down onto the posts. It's a fiddly job IIRC.
I leave the keys around the space bar (generally the open-apple and closed-apple keys) off until I get the space bar on. That leaves some room for fiddling the metal bar on. The bar slips into the base of the plastic posts.